1. Field of the Invention
This invention relates to a device for holding a part and a method for making electrical contact with the part. In particular, this invention relates to a chuck for holding a semiconductor wafer using a suction force and a probing method for making electrical contact with a device test pad on the semiconductor wafer.
2. Disclosure of Related Art
Chucks have long been used to fixedly hold semiconductor wafers during testing. Each semiconductor wafer may contain several thousand integrated circuits ("die") on a wafer surface that must be tested. Probers having a chuck and a probe card are utilized to make electrical contact with the integrated circuits so that electrical test signals can be transmitted from testers to the integrated circuits. The probe cards contain conductive probe needles that make contact ("probe") with force on conductive test pads disposed on each integrated circuit.
Prior to testing the integrated circuits on a wafer, the prober "profiles " the wafer which means the prober determines the top surface topology of the wafer. Profiling of the wafer in a plurality of locations is needed because the wafer may not be flat. Generally, integrated circuits have a protective outer passivation layer that may be made from materials that induce a positive or negative surface tension. The positive or negative surface tension may cause a relatively thin wafer to be concave or convex in shape. To compensate for the irregularity in wafer flatness, the prober stores the wafer profile in memory and utilizes the profile during the subsequent probing of all of the integrated circuits on the wafer.
In a first conventional chuck design, an electrostatic force between a chuck and a wafer is used to fixedly hold the wafer against the chuck. The electrostatic force is created by inducing a positive charge on the metal chuck and inducing a negative charge on the wafer. A problem associated with the first conventional chuck is that the metal chuck acts as an antenna and induces electrical noise into the wafers during testing. The induced electrical noise may cause erroneous integrated circuit test failures. Additionally, the metal chuck deflects from the force exerted by the probe needles during probing. Deflection of the chuck may result in a poor electrical contact between the integrated circuit test pads and the probe needles, resulting in erroneous test failures. Additionally, the deflection of the chuck results in the probe needles contacting and fracturing the passivation layer surrounding the test pad which results in scrapped die.
In a second conventional chuck design, a chuck with a top surface containing a plurality of increasingly larger concentric vacuum grooves is utilized to hold a wafer against the chuck utilizing a suction force. Each concentric vacuum groove has one corresponding vacuum hole. A problem associated with the second conventional chuck is that a concave or convex shaped wafer may not be pulled flat during the "profiling " of the wafer resulting in erroneous integrated circuit test failures. The second conventional chuck has a small number of vacuum holes resulting in a relatively small amount of air flow through the vacuum grooves. The small amount of air flow through the vacuum grooves results in a relatively small suction force being applied to the wafer that may be insufficient to hold the wafer flat against the chuck. During wafer probing, when the probe needles initially contact an integrated circuit in an area of the wafer disposed off of the chuck, the probe needles may move the wafer a sufficient vertical distance so that the small vacuum force applied by the chuck causes the wafer to be sucked flat against the chuck. The shape of the wafer during subsequent testing no longer matches the wafer profile stored in the prober memory. Thereafter, the prober utilizing the profile may move the probe needles an insufficient vertical distance during probing to make electrical contact with the test pads on the integrated circuits resulting in erroneous integrated circuit test failures.
FIG. 1 illustrates a semiconductor die 10 including a substrate 12, an aluminum test pad 14, and an oxide layer 16. Die 10 is disposed on a chuck 18 and a probe needle 20 is contacting test pad 14 on die 10 to remove a portion of oxide layer 16 that forms on test pad 14. Oxide layer 16 also known as "flash oxide", automatically forms when aluminum test pad 14 is exposed to oxygen. Oxide layer 16 acts as an insulator and a portion of oxide layer 16 must be removed in order for probe needle 20 to have an electrical conduction path to test pad 14. In one known test pad scrubbing process, the oxide layer on a test pad is removed by moving a probe needle relative to the test pad a plurality of movements while contacting the test pad. Initially, the probe needle contacts the test pad at a first location. The probe needle is subsequently moved to a plurality of locations on the test pad to approximate a circular scrubbing motion. A problem with the above-described scrubbing process is that in order to move the probe needle in a circular motion relative to the test pad, a large plurality of X and Y axis movements are needed. Using the above-mentioned probing process may result in substantial probing time being required during the testing of wafers.
There is thus a need for a device and method that minimizes or eliminates one or more of the above-mentioned deficiencies.